Maine DEP Monthly
O & M Newsletter

Maine and WEF's
Operation Forum

Penobscot Watershed and Development of a TMDL 


EPA Binational Toxics
Strategy

Maine Rural Water
Association


Maine Wastewater
Operator Certification
Guide


Maine Is Technology
Newsletter

Maine Wastewater Control Association



Maine
Wastewater Engineering
Firms
 

Lagoon Aeration - Coarse Bubble Static Tubes

ll aeration systems try to dissolve oxygen into the wastewater by creating bubbles that contain oxygen. As the bubbles move through the water, air and oxygen will diffuse into the bulk liquid by transferring across the bubbles surface. Course bubble static tubes create bubbles ( photo of coarse bubble aeration pattern to the left ) by pushing air through a series of baffles that break up the air flow into bubbles. The bubbles are then ejected into the water at the top of the tubes. From this point, they flow upward to the ponds surface. Along this critical path, the air in the bubbles must diffuse into the ponds liquid. Once the bubbles reach the ponds surface, it will be to late to transfer oxygen and all of the remaining air will be lost to the atmosphere.

    The static tube style aerator is shown to the right. The design of this system is inherently flawed because the air is released into the water several feet off the lagoon floor. This means that no aeration can occur in the lower half of the lagoons depth. Sludge deposits that build up in this zone have no hope of ever being re-suspended. They simply sit on the bottom and slowly decay under septic conditions. This leads to an even greater depletion of oxygen from the overlying lagoons liquid layer.

    This design is further flawed in shallow lagoons given the fact that oxygen transfer efficiency in a lagoon increases with every foot of depth. A lagoon with ten feet of depth would be losing half of its potential oxygen transfer efficiency by virtue of the fact that the diffusers sit so far off the floor. With coarse bubble aeration, maximum oxygen transfer efficiency (OTE) of 0.75% per foot of depth can be expected. This means that the OTE of a ten foot deep lagoon would be 7.5% if the air was released at the lagoon's bottom. However, only half of this efficiency, perhaps about 4% OTE, is realized since the air is discharged half way up the pond's depth. Put another way, this same ten foot deep lagoon could transfer twice as much air into the lagoon if the coarse bubble diffusers were closer to the pond's floor. As will be discussed further, this efficiency could be further increased by creating fine bubbles instead of coarse bubbles.

   For more information on the use of coarse bubble aeration one may consult the Water Pollution Control Federation's Manual of Practice FD - 13 entitled "Aeration".
 

Fine Bubble Aeration